Exciton Ground State Research Articles

Exciton-Exciton Interaction in Monolayer MoSe2 from Mutual Screening of Coulomb Binding.

Excitons in two-dimensional (2D) semiconductors are particularly exciting, as reduced screening and dimensional confinement foster their pronounced many-body interactions. Optical pumping is typically used to create excitons so as to study their properties, but at the same time such pumping can also create unbound charge carriers. This makes experimental determination of the exciton-exciton interactions difficult. Most importantly, the two effects of band gap renormalization and Coulomb screening on the individual exciton resonance energy counteract each other. Here by comparing the influences of exciton and electron density on the exciton ground and excited states energies of monolayer MoSe2 using photoluminescence spectroscopy, we are able to distinctly identify the screening of Coulomb binding by the neutral excitons and by charge carriers. The energy difference between exciton ground state (A-1s) and excited state (A-2s) red-shifts by 5.5 meV when the neutral exciton density increases from 0 to 4 × 1011 cm-2, in contrast to the blue shifts with the increase of either electron or hole density. This energy difference change is attributed to the mutual screening of Coulomb binding of neutral excitons, a many-body effect that is over 5 times magnitude stronger than the conventional estimate of exciton-exciton interactions. From this mutual Coulomb screening we extract an exciton polarizability of α2Dexciton = 2.5 × 10-17 eV(m/V)2. Our finding uncovers a mechanism that dominates the repulsive part of many-body interaction between neutral excitons.

Antimony composition impact on band alignment in InAs/GaAsSb quantum dots

We present a theoretical study of the electronic and excitonic states in InAs/GaAsSb quantum dots. We first center our study on the dependence of the antimony composition in the positioning of conduction- and valence-band alignments in InAs/GaAsSb/GaAs heterostructures. We predict a transition from type I to type II quantum dots at critical composition xc=0.128, which describes well the experimental trend. We discuss the influence of the quantum dot size and antimony composition on the spatial distributions of carriers and the exciton binding energy. We find that the ground state exciton binding energy is always significantly smaller ≃4meV for type II than for corresponding type I quantum dots ≃14meV. Finally, we also predict the excitonic radiative lifetime and find 1 ns for type I and 10 ns for type II quantum dots, in agreement with the existing experimental literature.

Polar magneto-optical Kerr effect spectroscopy with a microscope arrangement for studies on 2D materials.

We describe a setup for magneto-optical Kerr effect (MOKE) spectroscopy suitable for Kerr rotation (ϕ) and ellipticity (η) measurement on microscopic samples, such as flakes of two-dimensional materials. A spatial resolution of ∼25μm, limited by the demagnified monochromator exit slit image, was achieved. The use of mirrors allows for measurement in polar MOKE geometry with a conventional electro-magnet, without requiring holes in the magnet pole pieces. The microscope-like optics also has a 90° twisted periscope arrangement of two mirrors that helps transport light without change in its circular polarization state. A Jones matrix analysis of the setup brings out the influence of the beam-splitter on the measured signals. Its correction requires the ellipsometry parameters of the beam-splitter in transmission mode, which were measured separately. The working of the setup is tested by measuring the ϕ and η spectra of 2H-WS2 flakes at low temperature, verifying them using Kramers-Kronig analysis and extracting the Landé g-factor of the ground state exciton from them.

Identification of Semiconductor Nanocrystals with Bright Ground-State Excitons.

While semiconductor nanocrystals provide versatile fluorescent materials for light-emitting devices, their brightness suffers from the "dark exciton"─an optically inactive electronic state into which nanocrystals relax before emitting. Recently, a theoretical mechanism, the Rashba effect, was discovered that can overcome this limitation by inverting the lowest-lying levels and creating a bright excitonic ground state. However, no methodology is available to systematically identify materials that exhibit this inversion, hindering the development of superbright nanocrystals and their devices. Here, based on a detailed understanding of the Rashba mechanism, we demonstrate a procedure that reveals previously unknown "bright-exciton" nanocrystals. We first define physical criteria to reduce over 500,000 known solids to 173 targets. Higher-level first-principles calculations then refine this list to 28 candidates. From these, we select five with high oscillator strength and develop effective-mass models to determine the nature of their lowest excitonic state. We confirm that four of the five solids yield bright ground-state excitons in nanocrystals. Thus, our results provide a badly needed roadmap for experimental investigation of bright-exciton nanomaterials.

On the mechanical, thermoelectric, and excitonic properties of Tetragraphene monolayer

Two-dimensional carbon allotropes have attracted much attention due to their extraordinary optoelectronic and mechanical properties, which can be exploited for energy conversion and storage applications. In this work, we use density functional theory simulations and semi-empirical methods to investigate the mechanical, thermoelectric, and excitonic properties of Tetrahexcarbon (also known as Tetragraphene). This quasi-2D carbon allotrope exhibits a combination of squared and hexagonal rings in a buckled shape. Our findings reveal that tetragraphene is a semiconductor material with a direct electronic bandgap of 2.66eV. Despite the direct nature of the electronic band structure, this material has an indirect exciton ground state of 2.30eV, which results in an exciton binding energy of 0.36eV. At ambient temperature, we obtain that the lattice thermal conductivity (κL) for tetragraphene is approximately 118 W/mK. Young’s modulus and the shear modulus of tetragraphene are almost isotropic, with maximum values of 286.0N/m and 133.7N/m, respectively, while exhibiting a very low anisotropic Poisson ratio value of 0.09.

Theory of acoustic-phonon involved exciton spin flip in perovskite semiconductors

We present a theory of the acoustic phonon assisted spin-flip Raman scattering (SFRS), or resonant photoluminescence with the spin flip of a photoexcited exciton localized in a bulk cubic-phase perovskite semiconductor. We consider the spin-flip transitions between the ground-state exciton spin sublevels in external magnetic field B and discuss the variation of their probability rate and polarization selection rules with the increase of B. The transitions are treated as two-quantum processes with the virtual to and fro transfer of the electron in the electron-hole pair between the bottom and first excited conduction bands. The transfer occurs due to both the electron-hole exchange interaction and the electron-phonon interaction. The theoretical results allow one to distinguish the phonon assisted Raman scattering from (a) the resonant Raman scattering with the combined spin flip of the localized resident electron and hole and (b) the biexciton-mediated SFRS analyzed previously.

Phonon‐Bottleneck Enhanced Exciton Emission in 2D Perovskites

AbstractLayered halide perovskites exhibit remarkable optoelectronic properties and technological promise, driven by strongly bound excitons. The interplay of spin‐orbit and exchange coupling creates a rich excitonic landscape, determining their optical signatures and exciton dynamics. Despite the dark excitonic ground state, surprisingly efficient emission from higher‐energy bright states has puzzled the scientific community, sparking debates on relaxation mechanisms. Combining low‐temperature magneto‐optical measurements with sophisticated many‐particle theory, the origin of the bright exciton emission in perovskites is elucidated by tracking the thermalization of dark and bright excitons under a magnetic field. The unexpectedly high emission is clearly attributed to a pronounced phonon‐bottleneck effect, considerably slowing down the relaxation toward the energetically lowest dark states. It is demonstrated that this bottleneck can be tuned by manipulating the bright‐dark energy splitting and optical phonon energies, offering valuable insights and strategies for controlling exciton emission in layered perovskite materials that is crucial for optoelectronics applications.

Behavior of exciton in direct−indirect band gap Al x Ga1−x As crystal lattice quantum wells

Excitons have significant impacts on the properties of semiconductors. They exhibit significantly different properties when a direct semiconductor turns in to an indirect one by doping. Huybrecht variational method is also found to influence the study of exciton ground state energy and ground state binding energy in Al x Ga1−x As semiconductor spherical quantum dots. The Al x Ga1−x As is considered to be a direct semiconductor at Al concentration below 0.45, and an indirect one at the concentration above 0.45. With regards to the former, the ground state binding energy increases and decreases with Al concentration and eigenfrequency, respectively; however, while the ground state energy increases with Al concentration, it is marginally influenced by eigenfrequency. On the other hand, considering the latter, while the ground state binding energy increases with Al concentration, it decreases with eigenfrequency; nevertheless, the ground state energy increases both with Al concentration and eigenfrequency. Hence, for the better practical performance of the semiconductors, the properties of the excitons are suggested to vary by adjusting Al concentration and eigenfrequency

Spatially-indirect and hybrid exciton–exciton interaction in MoS2 homobilayers

Interlayer excitons in transition-metal dichalcogenide (TMD) bilayers, alongside their interplay with direct excitonic species, are supposed to offer a pathway towards robust nonlinearity, enabling the exploration of many-body quantum effects. We present a theoretical investigation of interaction among various exciton species within these structures where Coulomb attraction and repulsion are subject to reduced screening. For a homobilayer MoS2, we examine both direct, spatially-indirect, and hybridised excitons, considering the effects of direct and exchange interaction of electrons and holes distributed across one or different layers. Similar physics arises in perfectly aligned twisted TMD heterobilayers which support the direct-to-indirect exciton hybridisation. Deriving the exciton–exciton interaction matrix elements, we unveil a distinct non-monotonic dependence of the interaction on transferred momentum, changing sign from repulsive to attractive even for ground-state excitons, and compare our results with existing calculations for monolayers. Our findings demonstrate that for large momenta involved in high-density effects (strongly correlated phases), the interaction is chiefly governed by the prevailing attractive exchange component. At the same time, at small momenta that are more relevant for rarefied systems, we find that the enhancement of the interaction constant for dipolar species compared to intralayer non-dipolar excitons may be hindered by the surrounding medium. We draw comparisons with existing experiments and discuss the implications of our findings on the collective effects in TMD-based systems of excitons and exciton-polaritons.

Raman scattering excitation in monolayers of semiconducting transition metal dichalcogenides

Raman scattering excitation (RSE) is an experimental technique in which the spectrum is made up by sweeping the excitation energy when the detection energy is fixed. We study the low-temperature (T = 5 K) RSE spectra measured on four high quality monolayers (ML) of semiconducting transition metal dichalcogenides (S-TMDs), i.e. MoS2, MoSe2, WS2, and WSe2, encapsulated in hexagonal BN. The outgoing resonant conditions of Raman scattering reveal an extraordinary intensity enhancement of the phonon modes, which results in extremely rich RSE spectra. The obtained spectra are composed not only of Raman-active peaks, i.e. in-plane E′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${}^{{\\prime} }$$\\end{document} and out-of-plane A1′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${}_{1}^{{\\prime} }$$\\end{document}, but the appearance of 1st, 2nd, and higher-order phonon modes is recognized. The intensity profiles of the A1′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${}_{1}^{{\\prime} }$$\\end{document} modes in the investigated MLs resemble the emissions due to neutral excitons measured in the corresponding PL spectra for the outgoing type of resonant Raman scattering conditions. Furthermore, for the WSe2 ML, the A1′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${}_{1}^{{\\prime} }$$\\end{document} mode was observed when the incoming light was in resonance with the neutral exciton line. The strength of the exciton-phonon coupling (EPC) in S-TMD MLs strongly depends on the type of their ground excitonic state, i.e. bright or dark, resulting in different shapes of the RSE spectra. Our results demonstrate that RSE spectroscopy is a powerful technique for studying EPC in S-TMD MLs.

Promising TMDC-like optical and excitonic properties of the TiBr2 2H monolayer.

The presented simulation protocol provides a solid foundation for exploring two-dimensional materials. Taking the TiBr2 2H monolayer as an example, this material displays promising TMDC-like optical and excitonic properties, making it an excellent candidate for optoelectronic and valleytronic applications. The direct band gap semiconductor (1.19 eV) is both structurally and thermodynamically stable, with spin-orbit coupling effects revealing a broken mirror symmetry in the K and K' valleys of the band structure, as confirmed by opposite values of the Berry curvature. A direct and bright exciton ground state was found, with an exciton binding energy of 0.56 eV. The study also revealed an optical helicity selection rule, suggesting selectivity in the valley excitation by specific circular light polarizations.

Thermodynamic behavior of correlated electron-hole fluids in van der Waals heterostructures

Coupled two-dimensional electron-hole bilayers provide a unique platform to study strongly correlated Bose-Fermi mixtures in condensed matter. Electrons and holes in spatially separated layers can bind to form interlayer excitons, composite Bosons expected to support high-temperature exciton condensates. The interlayer excitons can also interact strongly with excess charge carriers when electron and hole densities are unequal. Here, we use optical spectroscopy to quantitatively probe the local thermodynamic properties of strongly correlated electron-hole fluids in MoSe2/hBN/WSe2 heterostructures. We observe a discontinuity in the electron and hole chemical potentials at matched electron and hole densities, a definitive signature of an excitonic insulator ground state. The excitonic insulator is stable up to a Mott density of ~0.8 × 1012 cm−2 and has a thermal ionization temperature of ~70 K. The density dependence of the electron, hole, and exciton chemical potentials reveals strong correlation effects across the phase diagram. Compared with a non-interacting uniform charge distribution, the correlation effects lead to significant attractive exciton-exciton and exciton-charge interactions in the electron-hole fluid. Our work highlights the unique quantum behavior that can emerge in strongly correlated electron-hole systems.

Structural, optoelectronic, excitonic, vibrational, and thermodynamic properties of 1T’-OsO2 monolayer via ab initio calculations

To unravel the structural, energetic stability, electronic, optical, excitonic, vibrational, and thermodynamic properties of monoclinic 1A’-OsO2 monolayer, we employed the first-principles calculations based on density functional theory (DFT) within the generalized gradient approximation (GGA) and the HSE06 hybrid functional, considering the norm-conserved pseudopotentials, and a combination of a tight binding plus BSE (TB+BSE) approach for the analysis of optical and excitonic properties at IPA and BSE levels. Our simulations demonstrate that the 1A’-OsO2 monolayer is a structurally and energetically stable semiconductor, and gives us a direct bandgap value, E(Γ→Γ), of 0.304, 0.254, and 1.119 eV, which were obtained through GGA-PBE, GGA-PBE+SOC, and HSE06-level of calculation, respectively. From the excitonic and optical properties, we observe that this system shows a large exciton binding energy of around 0.3 eV for the indirect ground state exciton, displaying an optical bandgap of 0.78 eV. We also show the use of light polarization as a mechanism to control the refractive index. The phonon dispersion and the infrared (IR) and Raman spectra were obtained, with its main peaks being assigned. Lastly, through thermodynamic potentials calculations, the Free energy (F) indicates that the synthesis of the 1A’-OsO2 monolayer would be spontaneous even at low temperatures. All theses properties demonstrate that the 1A’-OsO2 monolayer has potential applications in optoelectronic and thermal devices at the nanoscale.

Stable electroluminescence in ambipolar dopant-free lateral p–n junctions

Dopant-free lateral p–n junctions in the GaAs/AlGaAs material system have attracted interest due to their potential use in quantum optoelectronics (e.g., optical quantum computers or quantum repeaters) and ease of integration with other components, such as single electron pumps and spin qubits. A major obstacle to integration has been the unwanted charge accumulation at the p–n junction gap that suppresses light emission, either due to enhanced non-radiative recombination or due to inhibition of p–n current. Typically, samples must frequently be warmed to room temperature to dissipate this built-up charge and restore light emission in a subsequent cooldown. Here, we introduce a practical gate voltage protocol that clears this parasitic charge accumulation, in situ at low temperature, enabling the indefinite cryogenic operation of devices. This reset protocol enabled the optical characterization of stable, bright, dopant-free lateral p–n junctions with electroluminescence linewidths among the narrowest (<1 meV; <0.5 nm) reported in this type of device. It also enabled the unambiguous identification of the ground state of neutral free excitons (heavy and light holes) as well as charged excitons (trions). The free exciton emission energies for both photoluminescence and electroluminescence are found to be nearly identical (within 0.2 meV or 0.1 nm). The binding and dissociation energies for free and charged excitons are reported. A free exciton lifetime of 237 ps was measured by time-resolved electroluminescence, compared to 419 ps with time-resolved photoluminescence.

Exciton ground-state energy with full hole warping structure

Exciton ground-state energy with full hole warping structure

DNA Origami Assembled Nanoantennas for Manipulating Single-Molecule Spectral Emission.

The emission spectrum of a dye is given by the energy of all of the possible radiative transitions weighted by their probability. This spectrum can be altered with optical nanoantennas that are able to manipulate the decay rate of nearby emitters by modifying the local density of photonic states. Here, we make use of DNA origami to precisely place an individual dye at different positions around a gold nanorod and show how this affects the emission spectrum of the dye. In particular, we observe a strong suppression or enhancement of the transitions to different vibrational levels of the excitonic ground state, depending on the spectral overlap with the nanorod resonance. This reshaping can be used to experimentally extract the spectral dependence of the radiative decay rate enhancement. Furthermore, for some cases, we argue that the drastic alteration of the fluorescence spectrum could arise from the violation of Kasha's rule.

Excitonic topological order in imbalanced electron-hole bilayers.

Correlation and frustration play essential roles in physics, giving rise to novel quantum phases1-6. A typical frustrated system is correlated bosons on moat bands, which could host topological orders with long-range quantum entanglement4. However, the realization of moat-band physics is still challenging. Here, we explore moat-band phenomena in shallowly inverted InAs/GaSb quantum wells, where we observe an unconventional time-reversal-symmetry breaking excitonic ground state under imbalanced electron and hole densities. We find that a large bulk gap exists, encompassing a broad range of density imbalances at zero magnetic field (B), accompanied by edge channels that resemble helical transport. Under an increasing perpendicular B, the bulk gap persists, and an anomalous plateau of Hall signals appears, which demonstrates an evolution from helical-like to chiral-like edge transport with a Hall conductance approximately equal to e2/h at 35 tesla,where e is the elementarycharge and h is Planck's constant. Theoretically, we show that strong frustration from density imbalance leads to a moat band for excitons, resulting in a time-reversal-symmetry breaking excitonic topological order, which explains all our experimental observations. Our work opens up a new direction for research on topological and correlated bosonic systems in solid states beyond the framework of symmetry-protected topological phases, including but not limited to the bosonic fractional quantum Hall effect.

Size-Dependent Lattice Symmetry Breaking Determines the Exciton Fine Structure of Perovskite Nanocrystals.

The order of bright and dark excitonic states in lead-halide perovskite nanocrystals is debated. It has been proposed that the Rashba effect, driven by lattice-induced symmetry breaking, causes a bright excitonic ground state. Direct measurements of excitonic spectra, however, show the signatures of a dark ground state, bringing the role of the Rashba effect into question. We use an atomistic theory to model the exciton fine structure of perovskite nanocrystals, accounting for realistic lattice distortions. We calculate optical gaps and excitonic features that compare favorably with experimental works. The exciton fine structure splittings show a nonmonotonic size dependence due to a structural transition between cubic and orthorhombic phases. Additionally, the excitonic ground state is found to be dark with spin triplet character, exhibiting a small Rashba coupling. We additionally explore the effects of nanocrystal shape on the fine structure, clarifying observations on polydisperse nanocrystals.

Moiré Enhanced Potentials in Twisted Transition Metal Dichalcogenide Trilayers Homostructures.

The exploration of moiré superlatticesholds promising potential to uncovernovel quantum phenomena emerging from the interplay of atomic structure and electronic correlation . However, the impact of the moiré potential modulation on the number of twisted layers has yet to be experimentally explored. Here, this work synthesizes a twisted WSe2 homotrilayer using a dry-transfer method and investigates the enhancement of the moiré potential with increasing number of twisted layers. The results of the study reveal the presence of multiple exciton resonances with positive or negative circularly polarized emission in the WSe2 homostructure with small twist angles, which are attributed to the excitonic ground and excited states confined to the moiré potential. The distinct g-factor observed in the magneto-optical spectroscopy is also shown to be a result of the confinement of the exciton in the moiré potential. The moiré potential depths of the twisted bilayer and trilayer homostructures are found to be 111 and 212 meV, respectively, an increase of 91% from the bilayer structure. These findings demonstrate that the depth of the moiré potential can be manipulated by adjusting the number of stacked layers, providing a promising avenue for exploration into highly correlated quantum phenomena.

Strongly Correlated Exciton-Magnetization System for Optical Spin Pumping in CrBr3 and CrI3 .

Ferromagnetism in van der Waals systems, preserved down to a monolayer limit, attracted attention to a class of materials with general composition CrX3 (X=I, Br, and Cl), which are treated now as canonical 2D ferromagnets. Their diverse magnetic properties, such as different easy axes or varying and controllable character of in-plane or interlayer ferromagnetic coupling, make them promising candidates for spintronic, photonic, optoelectronic, and other applications. Still, significantly different magneto-optical properties between the three materials have been presenting a challenging puzzle for researchers over the last few years. Herewith, it is demonstrated that despite similar structural and magnetic configurations, the coupling between excitons and magnetization is qualitatively different in CrBr3 and CrI3 films. Through a combination of the optical spin pumping experiments with the state-of-the-art theory describing bound excitonic states in the presence of magnetization, weconcluded that the hole-magnetization coupling has the opposite sign in CrBr3 and CrI3 and also between the ground and excited exciton state. Consequently, efficient spin pumping capabilities are demonstrated in CrBr3 driven by magnetization via spin-dependent absorption, and the different origins of the magnetic hysteresis in CrBr3 and CrI3 are unraveled.